Imaging devices, such as ink jet printers, typically include one or more printheads that have ink jets from which drops of ink are ejected onto an image receiving surface. The image receiving surface may comprise recording media, such as paper, transparency, and the like, or may comprise an intermediate imaging member, such as a rotating drum or belt. The ink jets include actuators that generate mechanical forces to expel ink drops through an ink jet nozzle onto the image receiving surface in response to an electrical voltage signal, sometimes called a driving signal. In general, the amplitude, or voltage level, of the ink jet drive signals determines the amount of ink ejected in each drop.
In order to form color images on the image receiving surface, the printheads of an ink jet printer are provided with one or more colors or shades of ink for the ink jets to eject onto the image receiving surface. Multicolor images having variations in color beyond the colors of ink used in a printer may be achieved by selectively depositing ink drops at the potential drop locations by using known dithering or halftoning techniques.
In addition to colored inks, some printheads of an imaging device may be provided with substantially colorless or clear ink for ejection by ink jets onto the image receiving surface. Clear ink may be ejected by ink jets on top of printed images to form an overcoat that protects the images from being smeared and also to provide a desired level of gloss for the media. Clear inks may be also be selectively applied, such as by halftoning or dithering, for a number of additional reasons including, for example, to reduce gloss differential between different portions of an image or print, to provide spot glossing, to embed “invisible” images or data into media, and the like.
For optimum image quality, the drop mass of the drops emitted by the ink jets of the printheads should be substantially the same, especially for printheads, or ink jets of the printheads, that utilize the same color or shade of ink, including clear ink. As is known in the art, variations in ink jet performance may cause the drop mass of drops emitted by different ink jets to vary from ink jet to ink jet within a printhead. Such variations in ink jet performance may also result in the average drop mass output by the ink jets of a printhead to vary from printhead to printhead. In color images, such drop mass variations may result in image quality defects such as banding or streaking in the colors of the images produced. When using clear ink, banding and streaking may be less noticeable but may still adversely affect the glossiness of the images produced.
As part of a setup or maintenance routine, the ink jets of an imaging device typically undergo a normalization or calibration process so that the ink jets of an imaging device generate ink drops having consistent and uniform drop mass. Such normalization processes typically involve adjusting or modifying the voltage level of the drive signals for the ink jets so that the drops generated have a desired drop mass.
To enable normalization of the drop mass of the drops produced by the ink jets of an imaging device, a baseline drop mass for the drops generated by the ink jets must first be determined. In some previously known systems, the baseline drop mass of drops emitted by the ink jets of an imaging device was determined by printing onto a recording sheet, such as a transparency, and measuring the weight of the recording sheet before and after the ink is deposited onto the sheet. The weight difference between the printed and non-printed sheet corresponds to the total weight of the ink on the sheet which may then divided by the total number of drops printed onto the sheet to arrive at the average drop mass for the ink jets used to print onto the sheet. Based on the determined average drop mass using the printed sheets, the drive signals for actuating the ink jets may be calibrated to adjust the drop mass of the drops produced by a printhead to be within specifications. While such a method is capable of determining an average drop mass output by ink jets, such techniques are typically only available for use at the factory, not in the field. In addition, such a method may require several iterations and huge amounts of resources, i.e., time, transparencies, and ink, to calibrate the overall drop mass in a printhead.
Another method of determining drop mass that has been used in some previously known systems involves printing test patterns onto an image receiving surface and scanning the test pattern with an image sensor. Such image sensors typically include a light source for illuminating the test pattern and light detector for measuring a reflectance of light from the test pattern. The measured reflectance value of the test pattern may be correlated to a drop mass for the drops used to form the test pattern. The magnitude of the reflectance from the test may be correlated to drop mass values for the ink jets. If the detected drop mass is not within specifications, the voltage level, or amplitude, of one or more segments, or pulses, of the driving signal ink jet may be selectively adjusted (if needed so that each ink jet of a printhead emits drops having substantially the same drop mass.
Because such previously known normalization processes rely on reflectance measurements to determine drop mass, they require that the ink drops used to form the test patterns have some form of colorant that is capable of reflecting light in a detectable manner. Clear ink, however, does not reflect light in the same manner as colored ink. Consequently, in order to normalize the ink jets of a printhead unit intended for use with clear ink using the previously known reflectance based normalization process, the printhead unit would have to first be filled with a colored ink so it could be normalized and then cleaned so that it could be filled with the appropriate clear ink which poses the risk of color contamination of the clear ink.